Radio tuning system



July 8, 1941;- v, D. LANDON 2,248,242

, RADIO TUNING SYSTEM Filed. Jan. 2, 1940 Patented July 8, 1941 UNITEDSTATES PATENT OFFICE RADIO TUNING SYSTEM Vernon D. Landon, Haddonfield,N. J assignor to Radio Corporation of America, a corporation of Delaware7 Claims.

The present invention relates to radio tuning systems, and has for itsprimary object to provide an improved tuning system of the characterreferred to wherein two variably tunable circuits having magnetic coreVariable inductances are caused to track over a predetermined tuningrange.

It is also an object of the present invention to provide asuperheterodyne oscillator tracking system embodying magnetic coretuning whereby alignment between the oscillator and R.-F. circuits ismaintained over a mid-frequency portion of the tuning range as well asat the ends of said range.

It is a still further object, of the invention to provide asuperheterodyne oscillator tracking system embodying magnetic corevariable tuning for covering a plurality of tuning range with the sametuning inductances.

The invention will, however, be better under stood from the followingdescription when considered in connection with the accompanying drawing,and its scope is pointed out in the appended claims.

In the drawing,

Figure 1 is a schematic circuit diagram of a tuning system embodying theinvention, and

Figure 2 is a similar schematic circuit diagram showing a modificationof a portion of the circuit of Fig. 1, also embodying the invention.

Referring to Fig. 1, 5 is a tuning inductance of the solenoid type forthe antenna or R.-F. stage of a superheterodyne receiver and 6 is asimilar oscillator tuning inductance arranged to be tunable conjointlywith the first named inductance over a predetermined variable tuningrange or a plurality of such ranges by means including a movablemagnetic core I in the R.-F. stage inductance and a movable magneticcore 8 in the oscillator tuning inductance joined or connected asindicated by the dotted connection 9, in a manner to move jointly inunison. Additional means hereinafter referred to, such as shuntcapacitors for the inductances, further determine the ranges of theinductances 5 and 6.

With magnetic core tuning, as with other forms of variable tuning, theproblem in tracking two or more variable tuning circuits involves thelining up or frequency adjustment of the circuits at not only the highand low frequency ends of the tuning range, but also at one or moreintermediate points between the ends of the tuning range. Furthermore,as in a superheterodyne receiver, the tuning of the two or more circuitsby unitary control means involvescovering differing frequency ranges inthe several circuits. Thus, the oscillator may have a lower ratio offrequency ranges to cover than the R.-F. stage and in order to trackproperly a tuning system may require the use of means for modifying therate of change of inductance in one or the other of two or moresimultaneously variable tuning circuits.

For example, where two variable tuning ranges are provided between twocircuits, such as the R.-F. and oscillator circuits of a superheterodynereceiver covering a lower band of frequencies and a higher band offrequencies, such as the normal broadcast and short Wave bands, it hasbeen found that the circuits may be made to track effectively over thehigh frequency range with an oscillator inductance coil only slightlygreater in diameter than the R.F. coil, and over the 10W,- frequencyrange by utilizing an oscillator tuning inductance of considerablylarger diameter than the R..-F. tuning inductance. Also the tracking inthe mid-tuning portion of the range is corrected if a portion of theoscillator coil, preferably at the entering end of the coil, with regardto the core, is shunted by a capacitor, the value of which is adjustedto bring the tracking of the two circuits into alignment in that portionof the variable tuning range.

If desired the diameters of the two coils may be made the same and asmall fixed inductance may be inserted in series with the oscillatorinductance. The added series inductance produces about the same effectin shortening the oscillator tuning range than an increase in oscillatorcoil diameter would have produced.

In Fig. 1, the oscillator coil 6 is only slightly larger in diameterthan the R..-F. coil 5, so as to track over a high frequency range. Whenlarger shunt condensers are used so as to cover a lower frequency range,a further decrease in oscillator tuning range is required. Since thediameter cannot be changed, having been fixed by the requirements of thehigh frequency range, the required reduction in range is accomplished byinserting a series inductance as shown and de-' scribed hereinafter.

In the circuit of Fig. 1, the inductances 5 and 6 are provided withsuitable capacitors and switching means whereby they may be adjusted fortuning through the high and low frequency ranges referred to, also ashereinafter pointed out, and are connected in circuit with a mixer orconverter tube In having a signal input grid ll connected to the highpotential terminal l2 of the R.-F. inductance 5 and an oscillator gridl3 connected with the high potential terminal 14 of the oscillatorinductance 6 through the usual grid leak and capacitor I5.

The entering end of the inductance for the core 1, which is the lowpotential end of the inductance, is provided with a terminal l8connected to ground or chassis IT as is also a corresponding terminal I8on the inductance M.

For the high frequency tuning range, the R.-F. or antenna circuit isprovided with a shunt capacitor 2i) and a series capacitor 2! in circuitwith an antenna or signal collector 22. Connections with the capacitors2G and 2i are provided by switches 23 and 24 arranged to be operatedconjointly, as indicated by the dotted connection 25. Each of theswitches is provided with an alternative connection whereby, for the lowfrequency tuning range, a capacitor 25 of higher value is placed inseries with the antenna and a capacitor 25 also of higher value isplaced in shunt with the inductance 5 to add capacity for the lowertuning range as provided by the movable core 7.

In the oscillator circuit, which is of the Colpitts type by way ofexample, a pair of series connected capacitors 27 and 28 across theinductance 6 provide a tap connection 29 for the oscillator cathode 38and, in addition, provide for tuning the oscillator circuit at the highfrequency end of the tuning range. Movement of the core 8 into the coil6 lowers the frequency response of the oscillator circuit.

To simplify manufacture and reduce cost, it is preferable that similarcores of high perme ability be used in coils which are of substantiallyequal length and diameter. With this arrangement, however, it has beenfound that the oscillator may cover too great a frequency range and maybe lined up for tracking at only one frequency with the R.-F. stage.Referring to both Figs. 1 and 2, wherein like reference numerals areused for the same parts and circuits as in Fig. 1, if the oscillatorcoil diameter is increased to a suitable value, while maintaining theR.-F.

. or first circuit the same, a desired wide tuning range may be covered,but the oscillator may become too high in frequency and thus out .ofalignment in the mid-portion of the tuning range.

It has been found, however, that proper alignment for tracking may behad in the mid-portion of thetuning range by tapping one of .the coilsor inductances, such as the oscillator inductance, at substantiallyone-third of its length from the high frequency end, that is, theentering end, and connecting a capacitor in shunt with the coil betweensaid tap and said end. In Figs. 1 and 2, the tap is indicated at 32 andthe capacitor is indicated at 33, connected between the tap 32 and theterminal [8 across the lower portion of the inductance t. The capacitor33 is adjusted to a value such that alignment of the two circuits isprovided at the mid-tuning range, thereby completing the alignment ofthe circuits for high and low frequency tuning with the same coils. Inother words, for tuning through the high frequency range such as theshort wave broadcast range the two inductances 5 and 6 may be of thesame length and only slightly different diameter, with the same corematerial and size of core as for tuning through the low frequency range.The oscillator inductance in the present case or the inductance of thecircuit having the lower tuning range is increased in diameter asindicated at M, Fig. 2, sufficiently to change the rate of tuningvariation to that which is proper for maintaining the circuits inalignment with unicontrol movement of the several cores.

The diameter of the winding 6a in Fig. 2 is determined by the shape andlength of the core 8 which may be the same as in Fig. 1. In any case,the diameter of the two windings 5 and 5a difier by an amount sufficientto provide tracking between the two tuned circuits when simultaneouslytuned by magnetic or other suitable movable cores which are moved byuni-control means.

As hereinbefore referred to, if the oscillator coil diameter isincreased to a predetermined Value, the desired tuning range will beprovided by the oscillator, with the oscillator frequency slightly highin the mid-tuning range to produce a predetermined intermediatefrequency. This is corrected by the shunt capacitor 33 across a portionof the winding at the entering end of the winding, which as used hereinrefers to the end of the winding which the core enters and leaves intuning.

When a second tuning range at the lower frequency is desired, thewindings 5 and 6 of Fig. l and the winding'Ga in Fig. 2, may be shuntedby additional capacitors or capacitors of higher or lower values toproduce a second tuning range at a lower mean frequency. These areprovided in the present example by the capacitors 25 and 26 across theR.-F. coil 5 and by capacitors 35 and 36 connectable across thecapacitors 21 and 28 in the oscillator circuit by a switch 31 when movedto the alternate position from that shown in the drawing, closingcontacts 38 and 39 with a lead connection 4!] for the cathode circuit 4|of the oscillator tube 10.

At the same time a switch 42 operable jointly with the switch 3'! servesto connect an additional capacitor 43 in parallel with the capacitor '33across the tapped portion of the oscillator co-il thereby to correct thetuning and alignment of the oscillator with the R.-F. circuit in themidrange of the low frequency tuning band.

t will be noted that an inductance 44 is included in Fig. 1 in serieswith the capacitor 35 in shunt with the capacitor 21.

The switching arrangement of Fig. 1 adapts the tuning system forreceiving signals in a high frequency range, such as the short Wave bandand in a second lower frequency range, such as the present broadcastband, by increasing the values of the shunting capacitors of theoscillator and R.-F. circuits and by inserting a small inductance inseries with the oscillator coil which approximately permits tracking insubstantially the same manner as; with a tuning inductance in theoscillator of greatly increased diameter which would otherwise berequired to provide proper tracking between the two circuits in thelower frequency band;

In one embodiment of the invention, the tuning range was changed byincreasing the tuning capacitors approximately 10 to 1. In theoscillator of the present example, the capacity at 2'! from grid tocathode, the capacity at 28 from cathode to ground, and the capacity at33 from the tap to ground are increased by the addition of capacitors35, 36 and 43, respectively.

The additional cathode to ground capacitor 36 is adjusted for optimumoscillator operation by providing the proper feedback. The additionalcapacitor 35 from grid to cathode chiefly affects the tuning range, andthe capacitor 43 on the tap connection is adjusted to av value resultingin best tracking in the mid-range of the low frequency tuning band. Theseries inductance M is in series with the grid-cathode capacitor 35 andmakes it unnecessary to change the diameter of the oscillator coil forthe lower frequency tuning band while securing good tracking throughoutthe tuning range of the oscillator.

It has been found that the rate of change of frequency with coremovement varies rapidly as the coil is just entering the coil or windingand also when the core is substantially completely in the coil. Withtuning inductance or coils of different diameters in the severalcircuits, such as the R.-F. and oscillator circuits, as hereinbeforereferred to, the variation in frequency differs and tracking in theextreme limitation of a tuning range in any band of frequency may beimproved by a slight shortening of the length of one of the coils and aslight axial shift in position.

While the invention has been described in connection with the variableR.-F. and oscillator tuned circuits of a superheterodyne receiver as thepresent preferred embodiment of the invention, it should be understoodthat the invention is not limited thereto but may be applied to othermagnetic \core tuning circuits with unitary or conjoint control of thetuning.

In any case, however, the rate of change of frequency at the enteringend of the tuning inductance of any of the uni-controlled circuits maybe increased or :changed by tapping the inductance winding in spacedrelation to the entering end and connecting a capacitor of apredetermined value between the tap and the said end of the winding, andthe tracking of any two circuits may be improved by increasing thediameter of one of the variable inductance windings in the tuningcircuits while utilizing substantially the same cores, that is of thesame material and shape in each of the two windings for lower cost ofproduction.

The cathode of the oscillator is connected to ground I! through anindulctance winding ti and a cathode resistor 48 for the purpose ofproviding a high impedance path to ground for R.-F. currents whilemaintaining the D.-C. impedance that of the cathode resistor. This,however, forms no part of the present invention. Likewise, the outputcircuit of the oscillator comprising the anode 49 and the intermediateoutput transformer 50 is conventional and requires no furtherdescription.

I claim as my invention:

1. In a radio tuning system, the combination of two tunable signalcircuits each comprising an inductance winding, similar magnetic tuningcores for said windings adapted to enter one end of each winding and tomove therethrough, thereby'to provide variable tuning of said circuits,uni-control means for moving said cores conjointly in predeterminedrelation to each other, thereby to cause tracking in the tuning of saidcircuits, and a capacitor connected in shunt with a portion of at leastone of said windings adjacent the entering end thereof with respect tothe core movement to improve the tracking of said circuits in apredetermined portion of the tuning range of said system.

2. In a radio tuning system, the combination of two tunable signalcincuits each comprising an inductance winding, means providing a shuntcapacity across each of said windings for turn ng said circuits each toa predetermined frequency, similar magnetic tuning cores for saidwindings adapted to enter one end of each winding and to movetherethrough, uni-control means for moving said coresconjointly, andmeans providing capacity in shunt with a portion of at least one of saidwindings adjacent the entering end thereof with respect to core movementfor causing improved tracking of said circuits in a midfrequency range,one of said windings being of a larger diameter than the other forfurther improving the tracking of said circuits throughout apredetermined frequency range including said mid-frequency range. V

3. In a radio tuning system, the combination of two tunable signalcircuits each comprising an inductance winding, means providing a shuntcapacity across each of said windings for tuning said circuits each to apredetermined frequency, similar magnetic tuning cores for said windingsadapted to enter one end of each winding and to move therethrough, oneof said coils being displaced with respect to the entering end of thecore associated therewith, uni-control means for moving said coresconjointly into and through at least a portion of saidpoils, meansproviding capacity in shunt with a portion of at least one of saidwindings adjacent the entering end thereof with respect to core movementfor causing improved tracking of said circuits in a midfrequency range,one of said windings being of a larger diameter than t e other forfurther improving the tracking of said circuits through a predeterminedfrequency range including said mid-frequency range.

4. In a superheterodyne receiver, the combination with a tunable signalcircuit having a tuning inductance comprising a solenoid coil providedwith a movable magnetic core tuning element, of an oscillator circuitcomprising a second tuning inductance comprising a second solenoid coilof larger diameter than said first coil, a second like movable magneticcore tuning element for said last named inductance adapted to enter oneend of the said inductance in the tuning movement of the core element,and a capacitor connected in shunt with a portion of the second tuninginductance adjacent to said end.

5. In a superheterodyne receiver, the combination with a tunable signalcircuit having a tuning inductance provided with a movable magnetic coretuning element, of an oscillator circuit comprising a second tuninginductance, a second like movable magnetic core tuning element for saidlast named inductance adapted to enter one end of the said inductance inthe tuning movement of the core element, means providing a capacityconnected in shunt with a portion of the second tuning inductanceadjacent to said end, for tracking said circuits in a predeterminedportion of the tuning range thereof.

6. In a radio tuning system, the combination of two tunable signalcircuits each comprising a tuning inductance provided by a solenoidcoil, one of said coils being of larger diameter than the other, similarmagnetic core tuning elements for said coils adapted to be movedconjointly and to enter one end of each winding, thereby to vary thetuning of said circuits through predetermined frequency ranges, thedifference in diameter of said coils providing for tracking saidcircuits over the tuning range thereof with a predetermined uniformfrequency difference, and a capacitor connected in shunt with a portionof the coil of larger diameter adjacent the entering end thereof, saidcapacitor having a value such that the tracking of said circuits in amidportion of the tuning range is caused to conform with the tracking ofsaid circuits throughout the remainder of the tuning range thereof.

7. In a superheterodyne receiver, the combination with a tunable signalcircuit having a tuning inductance of the solenoid type provided with amovable magnetic core tuning element, of an oscillator circuitcomprising a second tuning inductance of the solenoid type, a secondlike movable magnetic core tuning element for said last-named inductanceadapted to enter one end of the said inductance in the tuning movementof the core element, means providing a capacity connected in shunt witha portion of the second tuning inductance adjacent to said end, meansproviding a tuning capacity for each of said indwctances connected inshunt therewith, a third tuning inductance, and means for simultaneouslyincreasing the value of said lastnamed capacities and said first-namedcapacity and inserting said third tuning inductance in series with saidsecond tuning inductance, thereby to change the tuning range of saidcircuits by a predetermined amount.

VERNON D. LANDON.

